In a hydraulic excavator as a construction machine, a boom and an arm, as well as an arm and a bucket, are swingably coupled by a pin.
In a coupling section coupled by the pin, namely in a pin hinge joint, lubricant oil such as grease is injected into between the pin and a boss to make the swinging motion smooth, as well to prevent the wear and noise caused by metal contact.
Further, a bearing seal called radial seal is used to prevent earth and the like from entering through between the pin and the boss, as well as to prevent the lubricant oil from leaking out from a clearance between the pin and the boss.
An example of such a bearing seal is described in, for example, U.S. Pat. No. 5,380,016.
The bearing seal as described in the document has, as shown in FIG. 43, an outer seal 1 called flex ring, an inner ring 2 called tension ring, and a inner seal 3 called seal ring. The outer seal 1 fits to a boss 4, and the inner seal 3 is attached so as to be brought into contact with a pin 5.
In the bearing seal, a fixing ring 6 called mounting is arranged on the outer periphery of the outer seal 1 to make the outer seal 1 easy to be fitted to the boss 4.
In the bearing seal as described in the document, when mounting the inner seal 3 to the pin 5, the inner ring 2 is extended in the radial direction so that the diameter thereof is enlarged, and the inner seal 3 is brought into press-contact with the pin 5 by fastening force of the inner ring 2 due to shrinkage. Thus, a large surface pressure (namely, seal surface pressure) of 20 N/cm, for example, in the radial direction is generated in the inner seal 3.
For this reason, a material having high tensile modulus of, for example, 300 to 14,000 MPa and high elongation of, for example, 5% or higher is used for the inner ring 2.
Since the seal surface pressure applied to the inner seal 3 is generated by the fastening force of the inner ring 2, the inner seal 3 is required to be provided with two seal lips of a seal lip 3a and a stabilizer lip 3b to improve the sealing performance.
If the seal lip is formed by one piece only, since the inner ring 2 will not evenly shrink in width direction (the axial direction of the pin 5), and the fall of the seal lip will occur, so that the seal lip will not be properly brought into press-contact with the pin 5, thus the sealing performance will deteriorate. By adding the aforementioned stabilizer lip 3b, the inner ring 2 can evenly shrink in width direction, and the seal lip can be properly brought into press-contact with the pin 5, thus good sealing performance in normal time can be obtained.
Since the seal lip 3 has the inner seal 3a and the stabilizer lip 3b as described above, when the pin 5 and the boss 4 rotate against each other, frictional force will be generated in two places, that is, the inner seal 3a and the stabilizer lip 3b, so that the frictional force becomes great.
The outer seal 1 is made strong to resist the great frictional force. In other words, if the frictional force becomes great, when the pin 5 and the boss 4 rotate against each other, a great rotating force acts on the outer seal 1. The outer seal 1 may be damaged if it is weak, therefore, as described above, the outer seal 1 is made strong so as not to be damaged.
There is a clearance between the boss 4 and the pin 5, and when subjected to vibration caused by impact, the pin 5 can move in the radial direction by an amount equal to the clearance.
If the inner seal 3 displaces in the radial direction following the movement (displacement) of the pin 5 in the radial direction, then the sealing performance can be maintained. However since the inner seal 3 is brought into press-contact with the pin 5 by the fastening force of the inner ring 2, and further, since the outer seal 1 is strong and difficult to be displaced in the radial direction, the inner seal 3 can not be displaced in the radial direction following the displacement of the pin 5 in the radial direction.
Thus, a gap will be generated between the seal lip 3a of the inner seal 3 and the pin 5 due to the displacement of the pin 5 in the radial direction, so that sealing performance will deteriorate, and earth and sand will enter from the seal section, and the lubricant oil will leak out.
In a construction machine such as a hydraulic excavator, since the vibration caused by impact is great, the pin 5 of the bearing seal of the pin hinge joint is repeatedly displaced in the radial direction at high speed.
When the pin 5 is displaced to one side in the radial direction (for example, to the upper side in FIG. 43), since the outer seal 1 is difficult to be displaced, one side of each of the inner seal 3 and the inner ring 2 (for example, the side of the upper half in FIG. 43) in the radial direction will be displaced due to being pressurized by the pin 5. On the other hand, since the speed of the displacement of the pin 5 is high, and further, since the outer seal 1 is difficult to be displaced, the other side of the inner seal 3 and the inner ring 2 (for example, the side of the lower half in FIG. 43) in the radial direction will delay in following the displacement of the pin 5, so that the seal lip 3a will fail in keeping press-contact state with the pin 5, and the seal lip 3a will detach from the pin 5.
Thus, the sealing performance of the bearing seal as described in the document is good in normal time, but bad when subjected to vibration caused by impact.
In this type of seal having a inner seal, since the grease is difficult to leak out from between the inner seal and the pin when performing grease up work, the completion of the grease up work can not be confirmed.
Further, when the sealing performance becomes bad, earth and sand will become easy to enter, and the grease will leak out, therefore the durability of the pin hinge joint will be lowered, and the grease has to be supplied frequently.
In view of the foregoing, it is an object of the present invention to provide a bearing seal not only whose sealing performance is good in normal time, but whose sealing performance can be prevented from being deteriorated when subjected to vibration caused by impact, and also, whether or not a grease or the like is properly injected can be confirmed.